3 files changed, 73 insertions, 0 deletions
diff --git a/cgithub/src/app.ts b/cgithub/src/app.ts
index ca79b1f..e34c7b1 100644
--- a/cgithub/src/app.ts
+++ b/cgithub/src/app.ts
@@ -5,6 +5,7 @@ import {
   getGitHubBlob,
   getGitHubCommits,
   getGitHubIssues,
+  getGitHubLanguages,
   getGitHubLatestCommit,
   getGitHubOwner,
   getGitHubPulls,
@@ -158,6 +159,11 @@ export function createApp(eta: Eta) {
     );
   });
 
+  app.get("/api/:owner/:repo/languages", async (c) => {
+    const { owner, repo } = c.req.param();
+    return tryRenderFragment(c, "_languages.eta", getGitHubLanguages(owner, repo));
+  });
+
   app.get("/:owner/:repo/releases/tag/:tag", async (c) => {
     const { owner, repo, tag } = c.req.param();
     return tryRender(c, "release.eta", getGitHubRelease(owner, repo, tag));
diff --git a/cgithub/src/scraper.test.ts b/cgithub/src/scraper.test.ts
index 56f14b0..5b5e78b 100644
--- a/cgithub/src/scraper.test.ts
+++ b/cgithub/src/scraper.test.ts
@@ -4,6 +4,7 @@ import {
   getGitHubBlob,
   getGitHubCommits,
   getGitHubIssues,
+  getGitHubLanguages,
   getGitHubLatestCommit,
   getGitHubPulls,
   getGitHubRefs,
@@ -219,6 +220,33 @@ describe("GitHub scraper", () => {
       assert.ok(data.commit.authors.length > 0);
     });
 
+    it("should fetch languages", async () => {
+      const data = await getGitHubLanguages("actions", "deploy-pages");
+
+      assert.strictEqual(data.repo.owner, "actions");
+      assert.strictEqual(data.repo.name, "deploy-pages");
+
+      assert.deepStrictEqual(data.languages, [
+        { name: "JavaScript", percentage: 100, color: "#f1e05a" },
+      ]);
+    });
+
+    it("should fetch languages for a multi-language repository", async () => {
+      const data = await getGitHubLanguages("rust-lang", "rust");
+
+      const rust = data.languages.find((language) => language.name === "Rust");
+      assert.ok(rust);
+      assert.ok(rust.percentage > 50);
+      assert.strictEqual(rust.color, "#dea584");
+
+      // The aggregate bucket comes back without a name.
+      const other = data.languages.find((language) => language.name === "Other");
+      assert.ok(other);
+
+      const total = data.languages.reduce((sum, language) => sum + language.percentage, 0);
+      assert.ok(Math.abs(total - 100) < 1);
+    });
+
     it("should fetch branches", async () => {
       const data = await getGitHubRefs("actions", "deploy-pages", "branches");
 
diff --git a/cgithub/src/scraper.ts b/cgithub/src/scraper.ts
index 39ad2cd..223839f 100644
--- a/cgithub/src/scraper.ts
+++ b/cgithub/src/scraper.ts
@@ -285,6 +285,17 @@ export interface GitHubLatestCommit extends GitHubCommon {
   commit: LatestCommit;
 }
 
+export interface Language {
+  // "Other" for the aggregate bucket GitHub reports without a name.
+  name: string;
+  percentage: number;
+  color: string | null;
+}
+
+export interface GitHubLanguages extends GitHubCommon {
+  languages: Language[];
+}
+
 export interface Ref {
   name: string;
   isDefault?: boolean;
@@ -722,6 +733,34 @@ export async function getGitHubLatestCommit(
   };
 }
 
+interface SidebarPayload {
+  languages?: {
+    languages: {
+      // Absent for the "Other" bucket.
+      name?: string;
+      percentage: number;
+      color?: string;
+    }[];
+  };
+}
+
+// The repository sidebar (languages, contributors, ...) is not rendered into
+// the repo page at all; GitHub's own UI lazily fetches it as JSON from here.
+export async function getGitHubLanguages(owner: string, repo: string): Promise<GitHubLanguages> {
+  const payload = await fetchGitHubJson<SidebarPayload>(`${owner}/${repo}/_sidebar`);
+
+  const languages: Language[] = [];
+  for (const entry of payload.languages?.languages ?? []) {
+    languages.push({
+      name: entry.name ?? "Other",
+      percentage: entry.percentage,
+      color: entry.color ?? null,
+    });
+  }
+
+  return { repo: { owner, name: repo }, languages };
+}
+
 interface BranchRefEntry {
   name: string;
   isDefault: boolean;